The degree of branching in a polyolefin has various implications in properties relevant to its end use. For example, long chain branching can affect the flexural properties, stiffness, and impact performance of thermoplastic polyolefins (TPOs) which are widely used in the automotive and construction industry due to their advantages in terms of reduced weight, recyclability, and plasticizer migration. Flex-to-stiffness balance affects the impact performance of TPOs, which is an important consideration in automobile applications such as car bumpers. Long chain branching can also influence the microstructure, such as average particle size and distribution, of a TPO (e.g., of ethylene-butene plastomers in a polypropylene matrix). This in turns affects the surface properties of the end product, such as paintability and ease of paint peel-off. While some of these properties can be adjusted by compounding or formulating the polyolefin with certain additives, this approach is less preferred for economic and efficiency reasons.
There is a need for controlling the degree of branching during the polymerization process. Chain transfer agents such as hydrogen are typically added to reaction mixtures to modify the molecular weight and control branching of the polymer. This invention provides a method to control branching in a copolymer and to affect the properties of thermoplastic polyolefin compositions containing the copolymer by controlling the amount of isobutylene and/or other C4 components present during polymerization. This method is particularly advantageous in making copolymers such as ethylene-butene copolymer since isobutylene and the other C4 components typically are already present in commercial supplies of butene (butene-1) and during the reaction process. Traditionally, isobutylene and other C4 components are often purged to avoid buildup in the system resulting from the recycling process. Under a method of this invention, the amount of these materials can be readily controlled to obtain the desired branching level and thermoplastic polyolefin composition properties by adjusting their concentrations in the recycle stream prior to returning to the reaction mixture.
References in the art include U.S. Pat. No. 7,163,989 and those cited therein, U.S. Pat. Nos. 3,726,843; 3,912,698; 4,444,922; 5,599,885; 6,881,800; and EP 0552945; WO 92/14766; WO 94/00500; and WO 98/02471.